Induction furnace having improved susceptor for use in the continuous production of carbonaceous fibrous materials

ABSTRACT

A graphite susceptor for use in a tube furnace during the production of a continuous length of a carbonaceous fibrous material. The graphite susceptor has a passageway extending therethrough which includes an entrance portion and an exit portion with the cross-sectional area of the entrance portion being of a smaller cross-sectional area than that of the exit portion. In a preferred embodiment of the invention the crosssectional area of the entrance portion has a rectangular configuration, and is adapted for use in the production of graphitic tapes consisting of a plurality of continuous graphite filaments.

United States Patent Ferment [54] INDUCTION FURNACE HAVING IMPROVED SUSCEPTOR FOR USE IN THE CONTINUOUS PRODUCTION OF CARBONACEOUS FIBROUS MATERIALS [72] Inventor: George R. Ferment, Dover, NJ.

[73] Assignee: Celanese Corporation, New York, NY. [22] Filed: June 16, 1970 [2]] Appl. No.: 46,675

[52] US. Cl. ..23/259.5, 23/277 R, 23/2091, 23/2094, 264/80, 264/29, 264/27, 219/1049, 219/1051, 13/27, 68/2 [51] Int. Cl. ..C0lb 31/07 [58] Field of Search ..23/259.5, 277, 2091 F; 264/80, 29, 27; 219/1049, 10.51; 13/27; 252/421; 68/2; 18/8 SS W wl k Xx [is] 3,656,910 [451 Apr. 18,1972

[56] References Cited UNITED STATES PATENTS 2,788,260 4/1957 Rick ..23/277 X 1,997,741 4/1935 Northrup.... .....2l9/10.49 1,763,229 6/ 1930 Fourment ..2l9/ 10.51 X

Primary Examiner-James H. Tayman, Jr. Attorney-Thomas J. Morgan, Charles B. Barris and Kenneth E. Macklin [57] ABSTRACT 12 Claims, 1 Drawing Figure 1 INDUCTION FURNACE I IAYING IMPROVED SUSCEP'IOR FOR USE IN THE CONTINUOUS PRODUCTION OF CARBONACEOUS FIBROUS MATERIALS BACKGROUND OF THE INVENTION In the search for high performance materials, considerable interest has been focused upon carbon fibers and apparatus capable of efficiently producing the same. The term carbon fiber as used herein is defined in. its generic sense as a carbonaceous fiber containing at least about 90 percent carbon by weight (preferably at least 9.5 percent carbon by weight) which may be formed of either graphite carbon or amorphous carbon. Graphite fibers are definedherein as carbon fibers which have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers or carbonized fibers, on the other hand, are defined as fibers which exhibit an essentially amorphous x-ray difiraction pattern. Graphite fibers generally have a much higher modulus and higher tenacity than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels, and ablative materials for heat shields on re-entry vehicles.

In the past one technique for the production of carbon fibers has employed the utilization of graphite tube furnaces. Such tube furnaces commonly comprise a cylindrical graphite tube (i.e. a susceptor), means for maintaining an inert gaseous atmosphere within the graphite tube, and means for heating the graphite tube to the highly elevated temperature required to produce the desired carbon fiber. For instance, the graphite tube may be heated inductively by placement within the windings of an induction coil, or heated by direct resistance heating. The fibrous material undergoing treatment has'been either statically positionedwithin the graphite tube or continuously passed through the same. By adjusting the relative length and location of the induction coil as well as the length of the cylindrical graphite tube, temperature gradients or profiles have been provided within the heating zone defined by the walls of the cylindrical graphite tube. Such temperature gradients have made possible, for instance, the continuous carbonization or carbonization and graphitization of a stabilized acrylic fibrous material.

The term graphite susceptor" as used herein is defined as a graphite structure containing a continuous passageway extending therethrough which is designed to be heated to an elevated temperature for the energy to the fibrous material present within the passageway.

In the past such susceptors have commonly been formed from one or more graphite tubes placed in an end-to-end relationship having a constant internal diameter. When susceptors of relatively large internal diameters (e.g. 2 or more inches) have been employed, difiiculty has been encountered in mainpurpose of imparting radiant taining the desired temperature profile within the interior of the susceptor.

It is an object of the invention to provide an improved graphite susceptor for use in the continuous production of carbon fibers.

It is an object of the invention to provide a graphite susceptor which may be utilized in the production of graphitic fibrous materials exhibiting enhanced tensile properties.

It is an object of the invention to provide a graphite susceptor in which the temperature gradient provided therein may be readily controlled.

. susceptor which is particularly useful It is'another object of the invention to provide a graphite susceptor wherein a, substantial quantity of the radiant energy present within the exit (e.g. graphitization) portion thereof is effectively retained therein and is not transferred to the en trance portion thereof.

It is another object of the invention to provide an adjustable graphite susceptor in which the temperature gradient produced therein may be varied.

It is a further object of the invention to provide a graphite susceptor which facilitates the production of continuous lengths of carbonaceous fibrous materials without the necessity to resort to extremely long susceptors, or relatively slow speeds for passage of the fibrous material through the susceptor.

It is a further object of the invention to provide a graphite in converting a relatively wide tape of a stabilized acrylic fibrous material to a graphitic form while retaining its original fibrous configuration essentially intact.

vThese and other objects as well as the scope, nature, and utilization of the invention will be apparent from the drawings, the following description, and appended claims.

SUMMARY OF THE INVENTION An elongated graphite susceptor suitable for defining the configuration of a heating zone of a tube furnace during the production of a continuous length of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material through said heating zone containing an inert gaseous atmosphere having a temperature gradient, said elon? gated graphite susceptor having a continuous passageway extending therethrough, said continuous passageway including an entrance portion and an exit portion with said continuous passageway of said entrance portion having a smaller crosssectional area than said continuous passageway of said exit portion.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view partially cut away of a graphite susceptor in accordance with the present invention.

FIG. 2 is a transverse sectional view enlarged 2X taken along the line 2-2 of the graphite susceptor of FIG. 1.

FIG. 3 is a side view partially cut away of another embodiment of a graphite susceptor in accordance with the present invention.

FIG. 4 is a transverse sectional view enlarged 2X taken along the line 4--4 of the graphite susceptor of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS The continuous length of fibrous material which is continuously passed through the elongated susceptor of the present invention is capable of undergoing carbonization or graphitization while retaining or preserving its original fibrous configuration essentiallyintact. The fibrous material which undergoes such thermal treatment may be formed by conventional techniques and maybe provided in a variety of physical configurations. For instance, the fibrous material may assume the configuration of a continuous length of a multifilament yarn, tape, tow, strand, cable, or similar fibrous assemblage. The preferred fibrous material is a continuous multifilament tape.

The fibrous material optionally may be provided with a twist which tends to improve the handling characteristics. For in stance, a twist of about 0.5 to 5 tpi, and preferably about 0.3

t to 1.0 tpi, may be imported to a multifilament yarn. Also, a

material which has been previously thermally stabilized (as described hereafter) is passed for an appropriate residence "time through a temperature gradient present within the susceptor having a maximum temperature generally below about 2,000 C., and most commonly a maximum temperature of about 900? to l,600 C. During such carbonization reaction an inert atmosphere such as nitrogen, argon, helium, etc. is provided within the susceptor. During the carbonization reaction when the amorphous carbon fibrous material is formed elements present in the thermally stabilized fibrous material other than carbon, such as hydrogen, oxygen, and nitrogen, are substantially expelled.

When one wishes to form a continuous length of a graphitic carbon fibrous material, the starting material may optionally contain at least about 90 percent carbon by weight, and exhibit an essentially amorphous x-ray diffraction pattern. As is known in the art, amorphous carbon fibrous materials suitable for graphitization may be formed by a variety of techniques. For instance, organic polymeric fibrous materials which are capable of undergoing thermal stabilization may be initially stabilized by treatment in an appropriate atmosphere at a moderate temperature (e.g. 200 to 400 C.), and subsequently heated in an inert atmosphere at a more highly elevated temperature, e.g. 900 to 1 ,000 C., or more, until a carbonized fibrous material is formed which exhibits an essentially amorphous x-ray diffraction pattern. The exact temperature and atmosphere utilized during the initial stabilization of an organic polymeric fibrous material commonly vary with the composition of the precursor as will be apparent to those skilled in the art. As described above, during the carbonization reaction elements present in the fibrous material other than carbon are substantially expelled. Suitable organic polymeric fibrous materials which are capable of undergoing thermal stabilization and subsequent carbonization include an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric materials are particularly suited for use in the formation of carbonaceous fibrous materials. lllustrative examples of suitable cellulosic materials include the natural and regenerated forms of cellulose, e.g. rayon. Illustrative examples of suitable polyamide materials include the aromatic polyamides, such as nylon 6T, which is formed by the condensation of hexamethylenediamine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly-2,2-m-phenylene-5,5-bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may be formed primarily of recurring acrylonitrile units. For

instance, the acrylic polymer should contain not less than about 85 mol percent of recurring acrylonitrile units with not more than about 15 mol percent of a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monovinyl compounds.

During the formation of a preferred carbonized starting material for graphitization in the elongated graphite susceptor of the present invention, a continuous length of acrylic fibrous material may be initially stabilized in an oxygen-containing atmosphere (i.e. preoxidized) on a continuous basis in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5, l968, of Dagobe rt E. Stuetz, which is assigned to the same assignee as the present invention and is herein incorporated by reference. More specifically, the acrylic fibrous material should be either an acrylonitrile homopolymer or an acrylonitrile copolymer which contains no more than about mol percent of one or more monovinyl comonomers copolymerized with acrylonitrile. In a particularly preferred embodiment of the invention the fibrous material is derived tion essentially" intact,"andv is non-burning when subjected to an ordinary match flame.

A continuous'length of the fibrous material capable of undergoing graphitization is continuously passed through a heating zone present withinthe graphite susceptor having a maximum temperature of at least 2,000, e.g. a maximum temperature of 2,000 to 3,l00 C. (preferably 2,400 to 3,l00 C.), containing an inert gaseous atmosphere for a residence time sufiicient to substantially convert the fibrous material to graphitic carbon while retaining its original fibrous configuration essentially intact. Suitable inert gaseous atmospheres for the heating zone include nitrogen, argon, helium, etc. For instance, a continuous length of an amorphous carbon fibrous material, e.g. a multifilament yarn, may be passed through the heating zone while at a graphitization temperature of at least 2,000 C. for a residence time of about 5 seconds to 4 minutes to produce graphitization. Longer graphitization heating times may be selected but generally yield no commensurate advantage. Preferred residence times while within about 50 C. of the maximum graphitization temperature commonly range from about 10 seconds to 200 seconds.

In a preferred graphitization technique a continuous length of a stabilized acrylic fibrous material (e.g. a tape) which is non-burning when subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith is continuously passed through the heating zone defined by the walls of the graphite susceptor containing an inert gaseous atmosphere and a temperature gradient in which the fibrous material is initially carbonized, and in which the carbonized fibrous material is heated to a maximum temperature of at least 2,000 C. until substantial graphitization occurs. Representative inert gaseous atmospheres for the heating zone in which both carbonization and graphitization are accomplished include nitrogen, argon, helium, etc.

When the fibrous material supplied to the heating zone is a stabilized acrylic fibrous material, it may be carbonized and graphitized while passing through a temperature gradient in accordance with procedures described in commonly assigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M. Clarke entitled Process for the Continuous Carbonization of a Stabilized Acrylic Fibrous Material; 17,780, filed Mar. 9, '1970 of Charles M. Clarke, Michael J. Ram, and John P. Riggs entitled Improved Process for the Carbonization of a Stabilized Acrylic Fibrous Material;" and 17,832, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and Arnold J. Rosenthal entitled Production of High Tenacity Graphitic Fibrous Materials. Each of these disclosures is herein incor porated by reference.

In accordance with a particularly preferred graphitization technique a continuous length of stabilized acrylic fibrous phitization heating zone defined by the walls of the graphite l susceptor containing an inert gaseous atmosphere and a temperature gradient in which said fibrous material is raised within a period of about 20 to about 300 seconds from about from'an acrylonitrile homopolymer. The stabilized acrylic fibrous material which is preoxidized in an oxygen-containing atmosphere is black in appearance, contains a bound oxygen content of at least 7 percent by weight as-determined by the Unterzaucher analysis, retains its original fibrous configura-' 800 C. to a temperature of about l,600 C. to form a continuous length of carbonized fibrous material, and in which said carbonized fibrous material is subsequently raised from about l,600 C. to a maximum temperature of at least about 2,400 C. within a period of about 3 to 300 seconds where it is maintained for about 10 seconds to about 300 seconds to form a continuous length of graphitic fibrous material.

The elongated graphite susceptor of the present invention is capable of utilization in the production of continuous lengths of carbonaceous fibrous materials of improved tensile properties. Heretofore the attainment of the desired temperature profile within the heating zone has been difficult to achieve particularly when the internal bore of the graphite susceptor has a relatively large cross-sectional area e.g. 2 inches or more in diameter). Such susceptors of relatively large cross-sectional area have heretofore been required when producing carbonaceous tapes of appreciable width. The elongated graphite susceptor of the present invention effectively makes possible the desired temperature profile within the heating zone defined by the walls of the susceptor, and avoids too rapid a heat-up rate. Too rapid a heat-up rate within the heating zone is believed to result in the uncontrolled evolution of volatiles from the fibrous material undergoing treatment, and to thereby generate flaws in the resulting fiber.

The elongated graphite susceptor of the present invention may be molded, machined or otherwise formed into an integral unit, or formed by the assembly of a plurality of components.

The elongated graphite susceptor of the present invention has a continuous passageway extending therethrough having an entrance portion and an exit (e.g. graphitization) portion. The cross-sectional area of the continuous passageway of the entrance portion is smaller than that of the cross-sectional area of the exit portion. The entrance portion of the passageway commonly extends about 30 to 70 percent of the total length of the continuous passageway, and has a cross-sectional area of about to 50 percent of that of the graphitization portion. The cross-sectional area of the continuous passageway of the entrance portion is preferably essentially rectangular, and the cross-sectional area of the continuous passageway of the exit portion is preferably essentially circular.

In the preferred embodiments of the present invention a graphite insert is positioned within the entrance end of an essentially cylindrical graphite tube so that it extends only a portion of the length of the tube and exhibits a configuration capable of substantially reducing the cross-sectional area of the bore of the graphite tube in the area where the insert is positioned. The graphite insert preferably has a length which is approximately 40 to 70 percent (e.g. 50 to 60 percent) of that of the cylindrical graphite tube. Additionally, approximately to 35 percent (e.g. to percent) of the graphite insert may protrude from the entrance end of the graphite tube.

If desired, the graphite insert may comprise a pair of Iongitudinal cylindrical graphite sections which are secured to the inner wall of the graphite tube. The outer diameter of the graphite cylinder from which the sections are obtained is essentially identical to that of the inner diameter of the graphite tube. As shown in FIGS. 3 and 4 (described hereafter), the sections may be of such a configuration that an essentially rectangular passageway is formed through which the fibrous material may be passed.

In a further embodiment of the invention the graphite insert is slidably positioned within the graphite tube. The temperature profile of the susceptor may accordingly be conveniently adjusted by varying the lengthof the insert protruding from the entrance end of the graphite tube.

FIGS. 1 and 2 illustrate a side view partially cut away, and a transverse sectional view enlarged 2X taken along the line 2 2 of a graphite susceptor 7 of the present invention. Essentially cylindrical graphite tube 1 has a length 72 inches, an outer diameter of 6 inches, and an inner diameter of 3.25 inches. The graphite tube 1 has an entrance end 2 and an exit end 4. A graphite insert 6 is positioned within the entrance end 2 of the graphite tube 1 and extends only a portion of the length of the graphite tube 1. The outer diameter of the graphite insert 6 is approximately 3.25 inches and frictionally engages the inner wall of the entrance end 2 of graphite tube 1. The total length of the graphite insert 6 is 40 inches, 8 inches of which protrude from the entrance end 2 of graphite tube 1.

A rectangular bore 8 extends the length of graphite insert 6 having a width of 2.5 inches and a height of 0.5 inch. A continuous passageway extends the length of the graphite susceptor 7 and has its entrance portion defined by the walls of rectangular bore 8, and its exit (e.g. graphitization) portion 10 defined by the inner wall of graphite tube 1 at the exit end 4.

FIGS. 3 and 4 illustrate a side view partially cut away, and a transverse sectional view enlarged 2 taken along the line 4 4 of another embodiment of a graphite susceptor 33 of the present invention. The essentially cylindrical graphite tube 20 has a length of 72 inches, an outer diameter of 6 inches, and an inner diameter of 3.25 inches. The graphite tube 20 has an end 22 and an exit end 24. A graphite insert comprising a pair of longitudinal cylindrical graphite sections 26 and 28 is positioned within the entrance end 22 of graphite tube 20 and extend only a portion of the length of the graphite longitudinal cylindrical graphite sections 26 and 28 were cut from a cylindrical graphite rod having an outer diameter of approximately 3.25 inches and have a total length of 42 inches, 12 inches of which protrude from the entrance end 22 of graphite tube 20. A continuous passageway extends the length of graphite susceptor 33. The entrance portion 34 of the passageway is defined by longitudinal cylindrical graphite sections 26 and 28 and the exposed portions 36 and 38 of the inner wall of graphite tube 20. The entrance portion 34 of the passageway is of an essentially rectangular configuration, having a width of approximately 3.25 inches and a height of 0.75 inch. The exit (e.g. graphitization) portion 40 of the passageway is defined by the inner wall of graphite tube 20 at the exit end 24.

The following examples are given as specific illustrations of the utilization of an elongated graphite susceptor of the present invention. It should be understood, however, that the conditions described in the examples may be varied, as will be apparent to those skilled in the art, and that the invention is not limited to the exact structures illustrated in the drawings.

EXAMPLE I A continuous length of 1,600 fil unwashed dry spun acrylonitrile homopolymer continuous filament yarn having a total denier of 1920 was selected as the starting material. The yarn was subjected to a healing treatment in which residual N,N-dimethyl formamide spinning solvent was evolved by passage for 10 minutes through a furnace provided with air at about 175 C. during which time the yarn shrank 10 percent in length. The yarn was continuously stabilized in accordance with the teachings of US. Ser. No. 865,332, filed Oct. 10, 1969, of Kenneth S. Burns and William M. Cooper, which is assigned to the same assignee as the present invention and is herein incorporated by reference. During the stabilization reaction (i.e. preoxidation) a plurality of lengths of the yarn were continuously passed while in a parallel relationship for about 135 minutes through a heat treatment oven provided with an air atmosphere at about 275 C. wherein the yarn was in a festooned relationship to each of a multiplicity of rotating parallel rolls. The resulting stabilized yarn was black in appearance, contained a bound oxygen content of about 10 percent by weight as determined by the Unterzaucher analysis, and was non-buming when subjected to an ordinary match flame.

The stabilized (i.e. preoxidized) yarn was stored in a forced air oven at C. while wound upon bobbins following stabilization. The yarn was next unwound from the bobbins and passed through a drying zone consisting of four 12 inch muffle furnaces placed in an end-to-end relationship and provided with circulating air at 200, 250, 300, and 340 C., respectively.

The dried yarn was next immediately passed to an Inductotherrn model Inducto I00 induction furnace provided with a I00 KW power source, a 24 turn water cooled copper coil, and the susceptor 7 illustrated in FIGS. 1 and 2. The copper coil was 36 inches in length, and was formed of copper tubing tube 20. The

' from which oxygen was substantially excluded.

The stabilized acrylonitrile yarn was continuously passed through the continuous passageway of graphite susceptor 7 at a rate of 0.5 meter per minute during which time it both carbonized and graphitized. The yarn was suspended within the interior of the passageway without contact with the walls of the passageway, and was under a longitudinal tension of 0.4 grams per denier. Carbonization resulted while the stabilized yarn was passed through rectangular bore 8 and graphitization resulted while the carbonized yarn passed through the graphitization portion 10 of the continuous passageway.

When passing through the heating zone defined by the graphite susceptor 7 containing a nitrogen atmosphere, the yarn was raised to a temperature of 800 C. in approximately 60 seconds after entering the susceptor, from 800 to 1,600 C. in approximately 30 seconds, and from l,600 C. to a maximum temperature of approximately 2,900 C. in approximately 40 seconds where it was maintained fl0 C. for approximately 48 seconds.

The resulting yarn exhibited a graphitic carbon x-ray diffraction pattern and a specific gravity of about 2.0. The following average single filament tensile properties were obtained for the graphite yam.

Tensile Strength Young's Modulus [2.8 grams per denier 3,540 grams per dcnier 028x si) (9l l0"psi) Tensile Strength Young's Modulus 10.] grams per denier 3,120 grams per denier (259Xl0" psi) (80 (l0 psi) EXAMPLE II Example I was repeated with the exception that the yarn was continuously passed through the continuous passageway of graphite susceptor 7 at a rate of 1.0 meter per minute instead of 0.5 meter per minute.

While passing through the heating zone defined by the graph-ite susceptor 7 containing a nitrogen atmosphere, the yarn was raised to a temperature of 800 C. in approximately 30 seconds after entering the susceptor, from 800 to l,600 C. in

approximately seconds, and from l,600 to a maximum temperature of approximately 2,900 C. in approximately seconds where it was maintained 0 C. for approximately 24 seconds.

The resulting yarn exhibited a graphitic carbon x-ray diffraction pattern and a specific gravity ofabout 2.0.The following average single filament tensile properties were obtained for the graphite yarn.

Tensile Strength Young's Modulus l2.2 grams per denier 3,260 grams per denier 3I2Xl0 si) 83xl0psi) In a comparative graphitization procedure the abovedescribed Example II was repeated with the exception that a graphite susceptor of a constant internal diameter was utilized. More specifically, the graphite susceptor was a graphite tube identical to that illustrated as graphite tube 1 in FIGS. 1 and 2, and graphite insert 6 was omitted.

The resulting yarn exhibited a graphitic carbon x-ray diffraction pattern and a specific gravity of about 2.0. The fol lowing average single filament tensile properties were obtained for the graphite yarn.

Tensile Strength Young's Modulus 10.6 grams per denier 3020 grams per denier (27lXl0 psi) (77Xl0'psi) Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations are to be considered within the purview and scope of the claims appended hereto.

I claim:

1. In a tube induction furnace suitable for the production of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material therethrough when supplied with an inert gaseous atmosphere comprising: an induction coil, an elongated graphite susceptor positioned within the windings of said induction coil having a continuous passageway extending therethrough which defines the configuration of the heating zone of said furnace, means for connecting said induction coil to a power source, and means for housing said induction coil and said susceptor; the improvement of providing an elongated graphite susceptor possessing a continuous passageway extending therethrough having an entrance portion and an exit portion and a solid wall which forms a continuous radial heat conductive path between its ex- 'ternal surface and its internal surface throughout its length in a configuration wherein said entrance portion of said passageway extends about 30 to 70 percent of the total length of said continuous passageway and has a cross-sectional area of about 10 to 50 percent of that of said exit portion, with said windings of said induction coil being substantially coextensive with said exit portion thereby being capable of producing a controlled temperature gradient within said heating zone having a higher temperature in said exit portion than in said entrance portion.

2. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material according to claim 1 wherein said continuous passageway of said entrance portion of said elongated graphite susceptor has an essentially rectangular cross-sectional area, and said continuous passageway of said exit portion of said elongated graphite susceptor has an essentially circular cross-sectional area.

3. in a tube induction furnace suitable for the production of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material therethrough when supplied with an inert gaseous atmosphere comprising: an induction coil, an elongated graphite susceptor positioned within the windings of said induction coil having a continuous passageway extending therethrough which defines the configuration of the heating zone of said furnace, means for connecting said induction coil to a power source, and housing means for said induction coil and said susceptor; the improvement of providing an elongated graphite susceptor possessing a continuous passageway extending therethrough having a solid wall which forms a continuous radial heat conductive path between its external surface and its internal surface throughout its length comprising:

a. an essentially cylindrical graphite tube having an entrance end and an exit end, and

b. a graphite insert positioned within the entrance end of said essentially cylindrical graphite tube and extending a portion of the length of said essentially cylindrical graphite tube having a configuration capable of substantially reducing the cross-sectional area of the heating zone defined by said essentially cylindrical graphite tube in the area where said insert is positioned to form an entrance portion of said passageway of a substantially reduced cross-sectional area with the remaining portion of said passageway serving as an exit portion, with said windings of said induction coil being substantially coextensive with said exit portion thereby being capable of producing a controlled temperature gradient within said heating zone having a higher temperature in said exit portion than in said entrance portion.

4. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert reduces the crosssectional area of the heating zone defined by said essentially cylindrical graphite tube to an essentially rectangular configuration.

5. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert has a length which is approximately 40 to 70 percent of that of said essentially cylindrical graphite tube.

6. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert has a length which is approximately 50 to 60 percent of that of said essentially cylindrical graphite tube.

7. An improved tube induction fumace suitable for the production of a carbonaceous fibrous material in accordance with claim 5 in which approximately to 35 percent of the length of said graphite insert protrudes from the entrance end of said essentially cylindrical graphite tube.

8. An improved tube induction furnace suitable for the production of carbonaceous fibrous material in accordance with claim 6 in which approximately to percent of the length of said graphite insert protrudes from the entrance end of said essentially cylindrical graphite tube.

9. In a tube induction furnace suitable for the production of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material therethrough when supplied with an inert gaseous atmosphere comprising: an induction coil, an elongated graphite susceptor positioned within the windings of said induction coil having a continuous passageway extending therethrough which defines the configuration of the heating zone of said furnace, means for connecting said induction coil to a power source, and housing means for said induction coil and said susceptor; the improvement of providing an elongated graphite susceptor possessing a continuous passageway extending therethrough having a solidwall which forms a continuous radial heat conductive path between its external surface and its internal surface throughout its length comprising:

a. an essentially cylindrical graphite tube having an entrance end and an exit end, and

b..a graphite insert positioned within the entrance end of said essentially cylindrical graphite tube and extending a portion of the length of said essentially cylindrical graphite tube having a configuration capable of substantially reducing the cross-sectional area of the heating zone defined by said essentially cylindrical graphite tube in the area where said insert is positioned to form an entrance portion of said passageway of a substantially reduced cross-sectional area having an essentially rectangular configuration with the remaining portion of said passageway serving as an exit portion, said graphite insert having a length which is approximately 40 to 70 percent of that of said essentially cylindrical graphite tube, and said graphite insert having approximately 15 to 35 percent of its length protruding from the entrance end of said essentially cylindrical graphite tube, with said windings of said induction coil being substantially coextensive with said exit portion thereby being capable of producing a controlled temperature gradient within said heating zone avmg a higher temperature in said exit portion than in said entrance portion.

10. An improved tube induction fumace suitable for the production of a carbonaceous fibrous material in accordance with claim 9 in which said graphite insert has a length which is approximately 50 to 60 percent of that of said essentially cylindrical graphite tube, and said graphite insert has approximately 20 to 30 percent of its length protruding from the entrance end of said essentially cylindrical graphite tube.

11. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 9 in which said graphite insert comprises a pair of longitudinal cylindrical graphite sections which are secured to the inner wall of said essentially cylindrical graphite tube.

12. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 10 in which said graphite insert comprises a pair of longitudinal cylindrical graphite sections which are secured to the inner wall of said essentially cylindrical graphite tube. 

2. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material according to claim 1 wherein said continuous passageway of said entrance portion of said elongated graphite susceptor has an essentially rectangular cross-sectional area, and said continuous passageway of said exit portion of said elongated graphite susceptor has an essentially circular cross-sectional area.
 3. In a tube induction furnace suitable for the production of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material therethrough when supplied with an inert gaseous atmosphere comprising: an induction coil, an elongated graphite susceptor positioned within the windings of said induction coil having a continuous passageway extending therethrough which defines the configuration of the heating zone of said furnace, means for connecting said induction coil to a power source, and housing means for said induction coil and said susceptor; the improvement of providing an elongated graphite susceptor possessing a continuous passageway extending therethrough having a solid wall which forms a continuous radial heat conductive path between its external surface and its internal surface throughout its length comprising: a. an essentially cylindrical graphite tube having an entrance end and an exit end, and b. a graphite insert positioned within the entrance end of said essentially cylindrical graphite tube and extending a portion of the length of said essentially cylindrical graphite tube having a configuration capable of substantially reducing the cross-sectional area of the heating zone defined by said essentially cylindrical graphite tube in the area where said insert is positioned to form an entrance portion of said passageway of a substantially reduced cross-sectional area with the remaining portion of said passageway serving as an exit portion, with said windings of said induction coil being substantially coextensive with said exit portion thereby being capable of producing a controlled temperature gradient within said heating zone having a higher temperature in said exit portion than in said entrance portion.
 4. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert reduces the cross-sectional area of the heating zone defined by said essentially cylindrical graphite tube to an essentially rectangular configuration.
 5. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert has a length which is approximatelY 40 to 70 percent of that of said essentially cylindrical graphite tube.
 6. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 3 in which said graphite insert has a length which is approximately 50 to 60 percent of that of said essentially cylindrical graphite tube.
 7. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 5 in which approximately 15 to 35 percent of the length of said graphite insert protrudes from the entrance end of said essentially cylindrical graphite tube.
 8. An improved tube induction furnace suitable for the production of carbonaceous fibrous material in accordance with claim 6 in which approximately 20 to 30 percent of the length of said graphite insert protrudes from the entrance end of said essentially cylindrical graphite tube.
 9. In a tube induction furnace suitable for the production of a carbonaceous fibrous material by the continuous passage of a continuous length of fibrous material therethrough when supplied with an inert gaseous atmosphere comprising: an induction coil, an elongated graphite susceptor positioned within the windings of said induction coil having a continuous passageway extending therethrough which defines the configuration of the heating zone of said furnace, means for connecting said induction coil to a power source, and housing means for said induction coil and said susceptor; the improvement of providing an elongated graphite susceptor possessing a continuous passageway extending therethrough having a solid wall which forms a continuous radial heat conductive path between its external surface and its internal surface throughout its length comprising: a. an essentially cylindrical graphite tube having an entrance end and an exit end, and b. a graphite insert positioned within the entrance end of said essentially cylindrical graphite tube and extending a portion of the length of said essentially cylindrical graphite tube having a configuration capable of substantially reducing the cross-sectional area of the heating zone defined by said essentially cylindrical graphite tube in the area where said insert is positioned to form an entrance portion of said passageway of a substantially reduced cross-sectional area having an essentially rectangular configuration with the remaining portion of said passageway serving as an exit portion, said graphite insert having a length which is approximately 40 to 70 percent of that of said essentially cylindrical graphite tube, and said graphite insert having approximately 15 to 35 percent of its length protruding from the entrance end of said essentially cylindrical graphite tube, with said windings of said induction coil being substantially coextensive with said exit portion thereby being capable of producing a controlled temperature gradient within said heating zone having a higher temperature in said exit portion than in said entrance portion.
 10. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 9 in which said graphite insert has a length which is approximately 50 to 60 percent of that of said essentially cylindrical graphite tube, and said graphite insert has approximately 20 to 30 percent of its length protruding from the entrance end of said essentially cylindrical graphite tube.
 11. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 9 in which said graphite insert comprises a pair of longitudinal cylindrical graphite sections which are secured to the inner wall of said essentially cylindrical graphite tube.
 12. An improved tube induction furnace suitable for the production of a carbonaceous fibrous material in accordance with claim 10 in which said graphite insert comprises a paIr of longitudinal cylindrical graphite sections which are secured to the inner wall of said essentially cylindrical graphite tube. 